Method and apparatus for validating a position in a satellite positioning system using range-rate measurements

ABSTRACT

Method and apparatus for validating an initial position in a satellite positioning system using range-rate measurements is described. In one example, range-rate measurements are obtained at the remote receiver with respect to a plurality of satellites. Expected range-rates are computed with respect to the plurality of satellites using the initial position. Single differences are computed using the range-rate measurements. Expected single differences are computed using the expected range-rates. Single difference residuals are computed between the single differences and the expected single differences. The single difference residuals are compared to a threshold. The initial position may be deemed valid if the absolute value of each of the single difference residuals is less than or equal to the threshold. A valid initial position may be used to fix the pseudorange integers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to satellite positioning systems and, moreparticularly, to a method and apparatus for validating a position in asatellite positioning system using range-rate measurements.

2. Description of the Background Art

Satellite Positioning System (SPS) receivers use measurements fromseveral satellites to compute position. SPS receivers normally determinetheir position by computing time delays between transmission andreception of signals transmitted from satellites and received by thereceiver on or near the surface of the earth. The time delays multipliedby the speed of light provide the distance from the receiver to each ofthe satellites that are in view of the receiver. Exemplary satellitepositioning systems include the Global Positioning System (GPS), theEuropean GALILEO system, and the Russian GLONASS system.

In GPS, each signal available for commercial use utilizes a directsequence spreading signal defined by a unique pseudo-random noise (PN)code (referred to as the coarse acquisition (C/A) code) having a 1.023MHz spread rate. Each PN code bi-phase modulates a 1575.42 MHz carriersignal (referred to as the L1 carrier) and uniquely identifies aparticular satellite. The PN code sequence length is 1023 chips,corresponding to a one millisecond time period. One cycle of 1023 chipsis called a PN frame or epoch.

GPS receivers determine the time delays between transmission andreception of the signals by comparing time shifts between the receivedPN code signal sequence and internally generated PN signal sequences.These measured time delays are referred to as “sub-millisecondpseudoranges,” since they are known modulo the 1 millisecond PN frameboundaries. If the data bit edges are identified for a particularsatellite, then the pseudorange is known modulo the data bit period, forexample, 20 ms in the current GPS system. Different satellite navigationsystems, and future changes in the GPS system may give different databit periods. In general, if a pseudorange is known modulo N ms, then werefer to it as a “fractional pseudorange”. By resolving the integernumber of milliseconds associated with each delay to each satellite,then one has true, unambiguous, pseudoranges. A set of four pseudorangestogether with knowledge of absolute times of transmission of the GPSsignals and satellite positions in relation to these absolute times issufficient to solve for the position of the GPS receiver. The absolutetimes of transmission (or reception) are needed in order to determinethe positions of the GPS satellites at the times of transmission andhence to compute the position of the GPS receiver.

Accordingly, each of the GPS satellites broadcasts a model of satelliteorbit and clock data known as the satellite navigation message. Thesatellite navigation message is a 50 bit-per-second (bps) data streamthat is modulo-2 added to the PN code with bit boundaries aligned withthe beginning of a PN frame. There are exactly 20 PN frames per data bitperiod (20 milliseconds). The satellite navigation message includessatellite-positioning data, known as “ephemeris” data, which identifiesthe satellites and their orbits, as well as absolute time information(also referred to herein as “GPS system time”) associated with thesatellite signal. The GPS system time information is in the form of asecond of the week signal, referred to as time-of-week (TOW). Thisabsolute time signal allows the receiver to unambiguously determine atime tag for when each received signal was transmitted by eachsatellite.

GPS satellites move at approximately 3.9 km/s, and thus the range of thesatellite, observed from the earth, changes at a rate of at most ±800m/s. Absolute timing errors result in range errors of up to 0.8 m foreach millisecond of timing error. These range errors produce a similarlysized error in the GPS receiver position. Hence, absolute time accuracyof 10 ms is sufficient for position accuracy of approximately 10 m.Absolute timing errors of much more than 10 ms will result in largeposition errors, and so typical GPS receivers have required absolutetime to approximately 10 milliseconds accuracy or better.

Another time parameter closely associated with GPS positioning is thesub-millisecond offset in the time reference used to measure thesub-millisecond pseudorange. This offset affects all the measurementsequally, and for this reason it is known as the “common mode error”. Thecommon mode error should not be confused with the absolute time error.As discussed above, an absolute time error of 1 millisecond leads torange errors of up to 0.8 meters while an absolute time error of 1microsecond would cause an almost unobservable range error of less than1 millimeter. A common mode error of 1 microsecond, however, results ina pseudorange error of 1 microsecond multiplied by the speed of light(i.e., 300 meters). Common mode errors have a large effect onpseudorange computations, and it is, in practice, very difficult tocalibrate the common mode error. As such, traditional GPS receiverstreat the common mode error as an unknown that must be solved for, alongwith position, once a sufficient number of pseudoranges have beenmeasured at a particular receiver.

Traditionally, the process of resolving the integer portions of thepseudoranges (“integer ambiguity resolution”) has traditionally requiredan initial estimate of receiver position that is close enough to thetrue receiver position for the integers to be uniquely defined. Notably,an initial position within 150 km of the true position will enableunambiguous resolution of the integers. In some cases, the only choicefor an initial position estimate at the receiver is the most recentlycomputed position, which is stored in a position cache. For example, thereceiver may be unable to communicate with any external source capableof providing a position estimate (e.g., a cellular telephone network).However, if the receiver has traveled more than 150 kilometers from thelast computed position, than the pseudorange integers will not bereliably resolved. If a position is computed using pseudoranges havingincorrect integers, the position will be invalid.

Accordingly, there exists a need in the art for a method and apparatusfor validating a position in a satellite position system.

SUMMARY OF THE INVENTION

Method and apparatus for locating position of a remote receiver isdescribed. In one embodiment, fractional pseudoranges are measured fromthe remote receiver to a plurality of satellites. An initial position isobtained at the remote receiver. For example, a recently computedposition stored in a position cache of the remote receiver may beselected as the initial position. A position of the remote receiver iscomputed using the fractional pseudoranges and the initial position.Range-rate measurements are obtained at the remote receiver with respectto the satellites. The computed position is validated using therange-rate measurements.

In another embodiment, fractional pseudoranges from the remote receiverto a plurality of satellites are measured. An initial position isobtained at the remote receiver. For example, a recently computedposition stored in a position cache of the remote receiver may beselected as the initial position. Range-rate measurements are obtainedat the remote receiver with respect to the plurality of satellites. Theinitial position is validated using the range-rate measurements. Theposition of the remote receiver is computed using the fractionalpseudoranges and the initial position in response to the initialposition being deemed valid. Notably, if the initial position is deemedvalid, the initial position may be used to fix the integer portions ofthe fractional pseudoranges.

BRIEF DESCRIPTION OF DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram depicting an exemplary embodiment of aposition location system;

FIG. 2 is a flow diagram depicting an exemplary embodiment of a methodfor locating position of a remote receiver in accordance with theinvention;

FIG. 3 is a flow diagram depicting an exemplary embodiment of a methodfor validating an initial position at a remote receiver in accordancewith the invention;

FIG. 4 is a flow diagram depicting another exemplary embodiment of amethod 400 for validating a position at a remote receiver in accordancewith the invention; and

FIG. 5 is a flow diagram depicting another exemplary embodiment of amethod 500 for locating position of a remote receiver in accordance withthe invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION OF THE INVENTION

A method and apparatus for validating a position in a satellitepositioning system (SPS) using range-rate measurements is described.Those skilled in the art will appreciate that the invention may be usedwith various types of mobile or wireless devices that are“location-enabled,” such as cellular telephones, pagers, laptopcomputers, personal digital assistants (PDAs), and like type wirelessdevices known in the art. Generally, a location-enabled mobile device isfacilitated by including in the device the capability of processingsatellite positioning system (SPS) satellite signals.

FIG. 1 is a block diagram depicting an exemplary embodiment of aposition location system 100. The system 100 comprises a remote receiver102 and a server 108. The remote receiver 102 is configured to receivesatellite signals from a plurality of satellites 112 in a constellationof satellites. The remote receiver 102 processes the received signals toproduce satellite measurement data (e.g., pseudoranges, range-ratemeasurements) with respect to the satellites 112. In one embodiment, theremote receiver 102 receives assistance data from the server 108. Theremote receiver 102 may communicate with the server 108 via a wirelessnetwork 110, a wired network 111, or both. Notably, the remote receiver102 may be configured for direct communication with the wired network111 or for indirect communication through a computer 113. The wirelessnetwork 110 may comprise any type of wireless network known in the art,such as a cellular telephone network. The wired network 111 may compriseany type of wired network known in the art, such as the Internet.

The remote receiver 102 may use the assistance data to aid inacquisition of the satellite signals and/or to compute position. Theassistance data may include satellite position information (e.g.,ephemeris data or other type of satellite orbit model), expected codephase, expected Doppler, a pseudorange model, and like type assistancedata known in the art, as well as any combination of such data. In oneembodiment, the remote receiver 102 computes its own position using thesatellite measurement data and the assistance data. Such a configurationis similar to the industry standard “Mobile Station Based” mode, butdiffers from the standards in that no initial position is required fromthe server. The management of the initial position by the remotereceiver is part of the current invention. In another embodiment, theremote receiver 102 sends the satellite measurement data to the server108 and the server 108 computes position of the remote receiver (e.g., amobile station assisted or MS-Assisted configuration).

Although the position location system 100 is shown as an Assisted GPS(A-GPS) system having a server, it is to be understood that the remotereceiver 102 may operate autonomously without receiving assistance datafrom the server 108. That is, in another embodiment, there is nocommunication between the remote receiver 102 and the server 108 and theremote receiver 102 does not receive assistance data. Instead, theremote receiver 102 receives satellite position information by decodingthe satellite signals to recover satellite navigation data using a wellknown decoding process. The remote receiver 102 then computes its ownposition using the satellite measurement data and the satellitenavigation data.

In one embodiment, the remote receiver 102 comprises a satellite signalreceiver 104, a wireless transceiver 106, a processor 122, supportcircuits 124, a communications transceiver 107, and a memory 120. Thesatellite signal receiver 104 receives satellite signals from thesatellites 112 using an antenna 116. The satellite signal receiver 104may comprise a conventional A-GPS receiver. An exemplary A-GPS receiveris described in commonly-assigned U.S. Pat. No. 6,453,237, issued Sep.17, 2002, which is incorporated by reference herein in its entirety. Thewireless transceiver 106 receives wireless signals from the wirelesscommunication network 110 via an antenna 118. The communicationstransceiver 107 may comprise a modem or the like for directcommunication with the wired network 111, or may comprise a serialtransceiver or the like for communicating with the computer 113.Although the remote receiver 102 is shown as having both a wirelesstransceiver and a communications transceiver, those skilled in the artwill appreciate that the remote receiver 102 may be configured with onlythe wireless transceiver 106 or only the communications transceiver 107.The satellite signal receiver 104, the wireless transceiver 106, and thecommunications transceiver 107 may be controlled by the processor 122.For purposes of clarity by example, the remote receiver 102 is shown asan assisted-SPS receiver. Those skilled in the art will appreciate,however, that the invention described herein may be used in aconventional autonomous SPS receiver (e.g., a receiver without awireless transceiver or a communications transceiver).

The processor 122 may comprise a microprocessor, instruction-setprocessor (e.g., a microcontroller), or like type processing elementknown in the art. The processor. 122 is coupled to the memory 120 andthe support circuits 124. The memory 120 may be random access memory,read only memory, removable storage, hard disc storage, or anycombination of such memory devices. The memory 120 may be used to storea cache of recently computed positions (“position cache 130”). Variousprocesses and methods described herein may be implemented via softwarestored in the memory 120 for execution by the processor 122.Alternatively, such processes and methods may be implemented usingdedicated hardware, such as an application specific integrated circuit(ASIC), or a combination of hardware and software. The support circuits124 include conventional cache, power supplies, clock circuits, dataregisters, I/O circuitry, and the like to facilitate operation of theremote receiver 102.

Satellite navigation data, such as ephemeris for at least the satellites112, may be collected by a network of tracking stations (“referencenetwork 114”). The reference network 114 may include several trackingstations that collect satellite navigation data from all the satellitesin the constellation, or a few tracking stations, or a single trackingstation that only collects satellite navigation data for a particularregion of the world. An exemplary system for collecting and distributingephemeris is described in commonly-assigned U.S. Pat. No. 6,411,892,issued Jun. 25, 2002, which is incorporated by reference herein in itsentirety. The reference network 114 may provide the collected satellitenavigation data to the server 108.

FIG. 2 is a flow diagram depicting an exemplary embodiment of a method200 for locating position of a remote receiver in accordance with theinvention. The method 200 begins at step 202. At step 204, fractionalpseudoranges are measured from the remote receiver 102 to a plurality ofsatellites (the satellites 112). In one embodiment, fractionalpseudoranges are obtained by measuring sub-millisecond pseudoranges atthe remote receiver 102. In another embodiment, fractional pseudorangesare obtained by synchronizing to the navigation data bit edges (e.g., inGPS, the navigation data bit edges occur every 20 milliseconds). Byidentifying the navigation data bit edges, the pseudoranges may bedetermined modulo the navigation data bit period (e.g., modulo 20milliseconds in GPS to provide sub-20 millisecond pseudoranges). In yetanother embodiment, fractional pseudoranges are obtained from sub-mspseudoranges for some satellites, and modulo the data bit period forother satellites. At step 206, an initial position of the remotereceiver 102 is obtained. For example, the remote receiver 102 may storeone or more recently computed positions in the memory 120 (e.g., theposition cache 130). The most recent of such positions may be selectedas an initial position. At step 208, range-rate measurements areobtained at the remote receiver 102 to validate the initial positionobtained at step 206. Exemplary processes for validating a positionusing range-rate measurements are described below with respect to FIGS.3 and 4.

At step 210, a determination is made as to whether the initial positionis valid. The validation process performed at step 210 is used todetermine if the initial position is within 150 km of the actualposition of the remote receiver (e.g., one half an epoch of a PN code).If so, the initial position may be used to fix the integer portions ofthe fractional pseudoranges. If not, the initial position cannot be usedto unambiguously fix the integer portions of the fractionalpseudoranges. Thus, if at step 210 the initial position is deemed to bevalid, the method 200 proceeds to step 212. At step 212, the pseudorangeintegers are resolved using the initial position obtained at step 206.If the initial position is deemed to be invalid at step 210, the method200 proceeds to step 214. At step 214, the pseudorange integers areresolved without using the initial position. An exemplary integerambiguity resolution process that may be performed without the need onan initial position is described in commonly-assign U.S. Pat. No.6,734,821, issued May 11, 2004, which is incorporated by referenceherein in its entirety. However, as described in U.S. Pat. No.6,734,821, in order to resolve the integers without an initial positionto within 150 kilometers, more than four fractional pseudorangemeasurements are required. In contrast, the integers may be fixed usingthe initial position at step 212 using only three or four pseudoranges.In some cases, the remote receiver 102 may not be capable of receivingmore than four satellite signals to compute pseudoranges (e.g., lowsignal-to-noise ratio environments). Thus, it is advantageous to resolvethe pseudorange integers using the initial position, if possible, atstep 212.

At step 216, position of the remote receiver 102 may be computed usingthe full pseudoranges and a navigation model in a well-known manner.Notably, in the general satellite navigation problem, there are nineunknowns:

-   -   Three position unknowns: x, y, z    -   Three velocity unknowns: {dot over (x)},{dot over (y)},ż    -   Three clock unknowns: t_(c), t_(s), f_(c)        where t_(c) is the common mode timing error (usually a        sub-millisecond value in GPS), t_(s) is the absolute time tag        error, and f_(c) is the frequency error in a local oscillator        within the remote receiver 102. One or more of the variables may        be known or estimated based on a-priori information (e.g., t_(s)        may known if the remote receiver 102 is calibrated to precise        GPS time). One or more of the unknown variables may be solved        for the pseudoranges and satellite orbit/clock data (e.g.,        ephemeris) in a well-known manner.

FIG. 5 is a flow diagram depicting another exemplary embodiment of amethod 500 for locating position of a remote receiver in accordance withthe invention. The method 500 begins at step 502. At step 504,fractional pseudoranges are measured from the remote receiver 102 to aplurality of satellites (e.g., sub-millisecond pseudoranges, modulo-20millisecond pseudoranges). At step 506, an initial position of theremote receiver 102 is obtained (e.g., a position from a positioncache). At step 508, the pseudorange integers are resolved using theinitial position. At step 510, position of the remote receiver 102 iscomputed using the full pseudoranges and a navigation model as describedabove. At step 512, range-rate measurements are obtained at the remotereceiver 102 to validate the position computed at step 510. Exemplaryprocesses for validating a position using range-rate measurements aredescribed below with respect to FIGS. 3 and 4. At step 514, adetermination is made as to whether the computed position is valid. Ifso, the method 500 ends at step 518. Otherwise, the method 500 proceedsto step 516. At step 516, the pseudorange integers are resolved withoutusing the initial position and the position of the remote receiver 102is re-computed using the pseudoranges. The method 500 then ends at step518.

FIG. 3 is a flow diagram depicting an exemplary embodiment of a method300 for validating a position at a remote receiver in accordance withthe invention. The method 300 may be performed at step 208 of theposition location method 200 of FIG. 2 to validate an initial positionand at step 512 of the position location method of FIG. 5 to validate acomputed position. The method 300 begins at step 302. At step 304,range-rate measurements are obtained at the remote receiver 102 withrespect to the satellites 112. The range-rate for a given satellite j isdenoted as {dot over (ρ)}_(j). In one embodiment of the invention, therange-rate measurements may be obtained by obtaining Dopplermeasurements with respect to the satellites 112. Doppler measurementsmay be made by the satellite signal receiver 104 in a well-known manner.Alternatively, the range-rate measurements may be obtained bydifferencing sets of fractional pseudoranges measured at two differenttimes (i.e., by computing the rate-of-change of pseudoranges or the“pseudorange rate”).

At step 306, expected range rates are computed using a position obtainedat step 305. For example, the position obtained at step 305 may be theinitial position obtained at step 206 in FIG. 2 or the computed positionobtained at step 510 in FIG. 5. The expected range rates may be computedby differencing sets of expected pseudoranges to the satellites 112based on the obtained position at two different times. The expectedrange-rate for a given satellite j, computed at an initial positionP_(i), is denoted as {dot over (ρ)}_(j) ^(i). At step 308, a referencesatellite is selected from the satellites 112 to be used insingle-difference computations. In one embodiment, the satellite withthe largest signal-to-noise ratio is selected as the referencesatellite. The subscript 0 is used to denote a reference range-ratemeasurement and a reference expected range-rate. Thus, the referencerange-rate measurement is denoted as {dot over (ρ)}₀, and the referenceexpected range-rate is denoted as {dot over (ρ)}₀ ^(i).

At step 310, single differences are computed between the range-ratemeasurement of the reference satellite and the remaining range-ratemeasurements. The single difference term is denoted by the delta symbol(Δ) and is used below to signify the difference between p, which is thejth range-rate measurement, and {dot over (ρ)}₀, which is the range-ratemeasurement for the reference satellite. Thus, the measured range-ratesingle difference for the jth satellite may be expressed as:

Δ{dot over (ρ)}_(j)={dot over (ρ)}_(j)−{dot over (ρ)}₀.

At step 312, single differences are computed between the expectedrange-rate of the reference satellite and the remaining expectedrange-rates. The expected range-rate single difference for the jthsatellite may be expressed as:

Δ{dot over (ρ)}_(j) ^(i)={dot over (ρ)}_(j) ^(i)−{dot over (ρ)}₀ ^(i).

At step 314, single-difference residuals are computed. The singledifference residual for the jth satellite (d_(j)) may be expressed as:

{dot over (ρ)}_(j)=Δ{dot over (ρ)}_(j)−Δ{dot over (ρ)}_(j) ^(i)

At step 316, each single difference residual is compared to a thresholdvalue and a determination is made as to whether each of the singledifference residuals satisfies the threshold. In one embodiment, theabsolute value of each single difference residual (having units ofmeters per second) is compared to a threshold. For example, a thresholdof 20 meters per second (m/s) may be established, which corresponds toan approximate 100 km difference between actual position and the initialposition. If the absolute value of a selected number (e.g., at leastone) of the single difference residuals is greater than the threshold(e.g., 20 m/s), then the position is deemed invalid. Conversely, if theabsolute value of a selected number (e.g., all) of single differenceresiduals is less than the threshold, then the position is deemed valid.Also, there may be an unexpected bias in each range-rate measurement ifthe remote receiver is moving. A threshold value of 20 m/s correspondsto a speed of approximately 72 km/h. Thus, if the speed of the remotereceiver 102 is unknown by more than 72 km/h, then the results of thetest performed at step 316 will be ambiguous (e.g., the initial positionmay be valid, but a false alarm is generated due to the unknown speed ofthe remote receiver 102.) In this case the problem is solved byre-computing speed using the range-rate measurements until speed isknown to better than 20 m/s. It is to be understood that other thresholdvalues may be chosen that are less or more restrictive than 20 m/s.

If the single difference residuals satisfy the threshold at step 316,the method 300 proceeds to step 318, where the position is flagged asbeing valid. If the single difference residuals fail the threshold, themethod 300 proceeds to step 320, where the position is flagged as beinginvalid. The method 300 ends at step 322.

FIG. 4 is a flow diagram depicting another exemplary embodiment of amethod 400 for validating a position at a remote receiver in accordancewith the invention. The method 400 may be performed at step 208 of theposition location method 200 of FIG. 2 to validate an initial positionand at step 512 of the position location method 500 of FIG. 5 tovalidate a computed position. The method 400 begins at step 402. At step404, range-rate measurements are obtained at the remote receiver 102with respect to the satellites 112. As described above, the range ratesmay be obtained by obtaining Doppler measurements or by measuring thepseudorange rate. At step 410, expected range-rates are computed using aposition obtained at step 406 and clock error data obtained at step 408.For example, the position obtained at step 406 may be the initialposition obtained at step 206 in FIG. 2 or the computed positionobtained at step 510 in FIG. 5. The clock error data obtained at step408 includes the relative clock error difference between the clockfrequency in the satellite signal receiver 104 and the clock frequencyin the satellites 112. The clock error data may be known at the remotereceiver 102 from a previously computed valid position and velocity.

At step 412, range-rate residuals are computed by differencing theexpected range-rates computed at step 410 and the measured range-ratesobtained at step 404. Since the clock error is known, the singledifference computation performed in the method 300 of FIG. 3 is notnecessary. At step 414, each range-rate residual is compared to athreshold value and a determination is made as to whether each of therange-rate residuals satisfies the threshold. In one embodiment, theabsolute value of each range-rate residual (having units of meters persecond) is compared to a threshold. If the absolute value of a selectednumber (e.g., at least one) of the range-rate residuals is greater thanthe threshold, then the position is deemed invalid. Conversely, if theabsolute value of a selected number (e.g., all) of range-rate residualsis less than the threshold, then the position is deemed valid. If therange-rate residuals satisfy the threshold at step 414, the method 400proceeds to step 416, where the position is flagged as being valid.Otherwise, the method 400 proceeds to step 418, where the position isflagged as being invalid. The method 400 ends at step 420.

In the preceding discussion, the invention has been described withreference to application upon the United States Global PositioningSystem (GPS). It should be evident, however, that these methods areequally applicable to similar satellite systems, and in particular, theRussian GLONASS system, the European GALILEO system, combinations ofthese systems with one another, and combinations of these systems andother satellites providing similar signals, such as the wide areaaugmentation system (WAAS) and SBAS that provide GPS-like signals. Theterm “GPS” used herein includes such alternative satellite positioningsystems, including the Russian GLONASS system, the European GALILEOsystem, the WAAS system, and the SBAS system, as well as combinationsthereof.

While the foregoing is directed to illustrative embodiments of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

1. A method of locating position of a remote receiver, comprising:measuring fractional pseudoranges from said remote receiver to aplurality of satellites; obtaining an initial position at said remotereceiver; computing position of said remote receiver using thefractional pseudoranges and said initial position; obtaining range-ratemeasurements at said remote receiver with respect to said plurality ofsatellites; and validating said position using said range-ratemeasurements.
 2. The method of claim 1, wherein said initial position isobtained from a memory in said remote receiver having at least onerecently computed position stored therein.
 3. The method of claim 1,wherein said step of obtaining range-rate measurements comprises:obtaining Doppler measurements with respect to said plurality ofsatellites.
 4. The method of claim 1, wherein said step of obtainingrange-rate measurements comprises: measuring additional fractionalpseudoranges from said remote receiver to a plurality of satellites atanother time; and differencing said fractional pseudoranges and saidadditional fractional pseudoranges to produce said range-ratemeasurements.
 5. The method of claim 1, wherein said step of validatingcomprises: computing expected range-rates with respect to said pluralityof satellites using said position; computing single differences usingsaid range-rate measurements; computing expected single differencesusing said expected range-rates; computing single difference residualsbetween said single differences and said expected single differences;and comparing said single difference residuals to a threshold.
 6. Themethod of claim 5, wherein said step of comparing comprises: determiningwhether an absolute value of each of said single difference residualssatisfies said threshold.
 7. The method of claim 6, further comprising:indicating validity of said position in response to the absolute valueof each of said single difference residuals being less than saidthreshold; and indicating invalidity of said position in response to theabsolute value at least one of said single difference residuals beinggreater than said threshold.
 8. The method of claim 5, wherein saidsteps of computing said single differences and computing said expectedsignal differences comprise: selecting a reference satellite from saidplurality of satellites, said reference satellite having an associatedreference range-rate measurement and reference expected range-rate insaid range-rate measurements and said reference range-rates,respectively; differencing said reference range-rate measurement andeach remaining one of said range-rate measurements to compute saidsingle differences; and differencing said reference expected range-rateand each remaining one of said expected range-rates to compute saidexpected single differences.
 9. The method of claim 8, wherein asignal-to-noise ratio at said remote receiver of said referencesatellite is larger than a signal-to-noise ratio of each remainingsatellite in said plurality of satellites.
 10. The method of claim 1,wherein said step of validating comprises: computing expectedrange-rates with respect to said plurality of satellites using saidposition and clock error data; computing range-rate residuals betweensaid range-rate measurements and said expected range-rates; andcomparing said range-rate residuals to a threshold.
 11. The method ofclaim 10, wherein said step of comparing comprises: determining whetheran absolute value of each of said range-rate residuals satisfies saidthreshold.
 12. The method of claim 11, further comprising: indicatingvalidity of said position in response to the absolute value of each ofsaid range-rate residuals being less than said threshold; and indicatinginvalidity of said position in response to the absolute value at leastone of said range-rate residuals being greater than said threshold. 13.The method of claim 1, wherein said step of computing said positioncomprises: fixing integer portions of said fractional pseudoranges usingsaid initial position.
 14. A method of locating position of a remotereceiver, comprising: measuring fractional pseudoranges from said remotereceiver to a plurality of satellites; obtaining an initial position atsaid remote receiver; obtaining range-rate measurements at said remotereceiver with respect to said plurality of satellites; validating saidinitial position using said range-rate measurements; and computingposition of said remote receiver using said fractional pseudoranges andsaid initial position in response to said initial position being deemedvalid.
 15. The method of claim 14, wherein said initial position isobtained from a memory in said remote receiver having at least onerecently computed position stored therein.
 16. The method of claim 14,wherein said step of obtaining range-rate measurements comprises:obtaining Doppler measurements with respect to said plurality ofsatellites.
 17. The method of claim 14, wherein said step of obtainingrange-rate measurements comprises: measuring additional fractionalpseudoranges from said remote receiver to a plurality of satellites atanother time; and differencing said fractional pseudoranges and saidadditional fractional pseudoranges to produce said range-ratemeasurements.
 18. The method of claim 14, wherein said step ofvalidating comprises: computing expected range-rates with respect tosaid plurality of satellites using said initial position; computingsingle differences using said range-rate measurements; computingexpected single differences using said expected range-rates; computingsingle difference residuals between said single differences and saidexpected single differences; and comparing said single differenceresiduals to a threshold.
 19. The method of claim 18, wherein said stepof comparing comprises: determining whether an absolute value of each ofsaid single difference residuals satisfies said threshold.
 20. Themethod of claim 19, further comprising: indicating validity of saidinitial position in response to the absolute value of each of saidsingle difference residuals being less than said threshold; andindicating invalidity of said initial position in response to theabsolute value at least one of said single difference residuals beinggreater than said threshold.
 21. The method of claim 18, wherein saidsteps of computing said single differences and computing said expectedsignal differences comprise: selecting a reference satellite from saidplurality of satellites, said reference satellite having an associatedreference range-rate measurement and reference expected range-rate insaid range-rate measurements and said reference range-rates,respectively; differencing said reference range-rate measurement andeach remaining one of said range-rate measurements to compute saidsingle differences; and differencing said reference expected range-rateand each remaining one of said expected range-rates to compute saidexpected single differences.
 22. The method of claim 21, wherein asignal-to-noise ratio at said remote receiver of said referencesatellite is larger than a signal-to-noise ratio of each remainingsatellite in said plurality of satellites.
 23. The method of claim 14,wherein said step of validating comprises: computing expectedrange-rates with respect to said plurality of satellites using saidinitial position and clock error data; computing range-rate residualsbetween said range-rate measurements and said expected range-rates; andcomparing said range-rate residuals to a threshold.
 24. The method ofclaim 23, wherein said step of comparing comprises: determining whetheran absolute value of each of said range-rate residuals satisfies saidthreshold.
 25. The method of claim 24, further comprising: indicatingvalidity of said initial position in response to the absolute value ofeach of said range-rate residuals being less than said threshold; andindicating invalidity of said initial position in response to theabsolute value at least one of said range-rate residuals being greaterthan said threshold.
 26. The method of claim 14, wherein said step ofcomputing said position comprises: fixing integer portions of saidfractional pseudoranges using said initial position.
 27. Apparatus forlocating position, comprising: a satellite signal receiver for measuringfractional pseudoranges to a plurality of satellites and for obtainingrange-rate measurements with respect to said plurality of satellites; amemory for storing an initial position; and a processor for computingposition using said fractional pseudoranges and said initial positionand for validating said position using said range-rate measurements. 28.Apparatus for locating position, comprising: a satellite signal receiverfor measuring fractional pseudoranges to a plurality of satellites andfor obtaining range-rate measurements with respect to said plurality ofsatellites; a memory for storing an initial position; and a processorfor validating said initial position using said range-rate measurementsand for computing position using said fractional pseudoranges and saidinitial position in response to said initial position being deemedvalid.